1. Field of the Invention
This invention relates to a methacrylic resin molding material and to a method for the production thereof, and more particularly to a methacrylic resin molding material for the manufacture of a cross-linked shaped article and to a method for the production thereof.
2. Description of Prior Art
Generally, the methacrylic resin formed predominantly of methyl methacrylate enjoys excellent weatherability and outstanding transparency and, because of this salience, finds utility in applications to covers for lamps, parts for automobiles, signboards, ornamental articles and sundry goods. Since the aforementioned methacrylic resin is a linear polymer, however, it has some disadvantages that it is deficient in resistance to heat, resistance to solvents, resistance to shocks, and surface hardness. Regarding the resistance to heat, for example, the methacrylic resin withstands heat up to about 100.degree. C. at most and, therefore, falls short of satisfying the needs arising from various applications for which resistance to heat is a requisite. In the case of parts for automobiles, the methacrylic resin cannot be used in covers for head lamps. In the case of tail lamps, since the amount of heat generated is growing because of increase in lamp size and increase in illuminance and the wall thickness of the cover is decreased to permit desired cost cut, the necessity of the covers of the lamps acquiring improved heat resistance is finding approval. The methacrylic resin is expected to find growing utility in applications to parts such as covers for meters on automobiles and two-wheelers (motorcycles) and covers for water heaters using solar energy which by nature experience great elevation of temperature under direct exposure to the sunlight. Thus, the development of a methacrylic resin composition capable of withstanding heat above the boiling point of water is longed for.
For the purpose of conferring improved heat resistance upon the methacrylic resin, there have been proposed numerous methods, e.g. a method resorting to copolymerization of methyl methacrylate with .alpha.-methyl styrene (U.S. Pat. No. 3,135,723), a method resorting to copolymerization of methyl methacrylate with .alpha.-methyl styrene and maleic anhydride (Japanese patent publication No. SHO 45(1970)-31,953 and No. SHO 49(1974)-10,156), and a method resorting to copolymerization of methyl methacrylate with .alpha.-methyl styrene and maleimide (Japanese patent laid-open No. SHO 48(1973)-95,490).
Indeed these methods are invariably capable of improving the methacrylic resin in heat resistance. Some of them, however, suffer from notably slow rate of polymerization or insufficient increase of conversion and, consequently, fail to obtain high conversion. Others, though capable of efficiently producing polymers rather quickly, suffer their products to assume color heavily and acquire transparency, weatherability, surface hardness and mechanical strength at levels lower than normally tolerated. Combining both merits and demerits, the conventional methods fall for short of being commercially feasible and have not been reduced to practice.
It is generally conceivable that introduction of a cross-linked configuration enables a polymer to acquire enhanced resistance to heat and to solvents. Since a cross-linked polymer has already formed a three-dimensional structure, it no longer can be molded by injection molding, extrusion molding or transfer molding. When the cross-linked polymer is molded by cast molding, it is liable to give rise to voids and bubbles. Thus, the cast molding of this cross-linked polymer fails to produce molded articles of complicated shape and suffers from inferior productivity.
Kodama et al., in their report inserted in Polymer Chemistry, Vol. 27, No. 297, p. 65, offer a statement, reading: "As one approach to the improvement of moldability of the cross-linked polymer, a method which comprises selecting a monomer having functional groups separated by a long distance from each other, setting this monomer polymerization while a fairly large proportion of the monomer still remains unaltered (above 90% of conversion), molding the first-stage polymer in a given shape, and thereafter performing the second-stage polymerization of the molded polymer until completion of the polymerization is conceivable." Despite this suggestion, they offer no working example of this method. In our repetition of the method described by Kodama et al., when the product of the first-stage polymerization was compression molded at elevated temperature in a metal die, the sample solidified and failed to flow and finally converted into a powder, thus showing absolutely no moldability. The term "molding" as used by Kodama et al. in the aforementioned statement means a very simple process of bending. It is not essentially synonymous with the molding process involving fluidity as aimed at by this invention.
An object of this invention, therefore, is to provide a novel methacrylic resin molding material and a method for the production thereof.
Another object of this invention is to provide a methacrylic resin molding material for manufacture of cross-linked molded articles and a method for the production of the resin material.
A further object of this invention is to provide a methacrylic resin molding material for manufacture of cross-linked molded articles excelling in physical properties such as heat resistance and resistance to solvents without any sacrifice of transparency and a method for the production of the molding material.
Still another object of this invention is to provide a methacrylic resin molding material of excellent storability for manufacture of cross-linked molded articles excelling in physical properties such as heat resistance and resistance to solvents without any sacrifice of transparency and a method for easy production of the molding material.